Imagers typically consist of an array of pixel cells containing photosensors, where each pixel cell produces a signal corresponding to the intensity of light impinging on that pixel cell when an image is focused on the array. These signals may then be stored, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically phototransistors, photoconductors, photogates or photodiodes. The magnitude of the signal produced by each pixel cell, therefore, is proportional to the amount of light impinging on the photosensor.
To allow the photosensors to capture a color image, the photosensors must be able to separately detect photons of wavelengths of light associated with at least first, second, and third colors, e.g., red (R), green (G), and blue (B) photons. Accordingly, each pixel cell must be sensitive only to one color or spectral band. For this, a color filter array is typically placed between the image being captured and the pixel cells so that each pixel cell measures the light of the color of its associated filter.
Color imaging typically requires three pixel cells (R, G, B) for the formation of a single color pixel cell. For example, a conventional color pixel cell array 50 is illustrated in FIG. 1 in a linear layout, for convenience, as including a red active pixel cell 52, a green active pixel cell 54, and a blue active pixel cell 56, spaced apart on a semiconductor substrate 16 by isolation regions 19. Each of the red, green, and blue active pixel cells 52, 54, 56 have respective red, green, and blue filters 17R, 17G, 17B, which are part of color filter array 117, which allow only red, green, and blue photons, respectively, to pass through. In practice, the color pixel cells are typically arranged in a Bayer pattern in rows and columns, with one row of alternating green and blue pixel cells, and another row alternating red and green pixel cells.
A brief description of the structural and functional elements of each of the red, green, and blue active pixel cells 52, 54, 56 is now provided. Each of the pixel cells 52, 54, 56 is shown in part as a cross-sectional view of a semiconductor substrate 16, which may be a p-type silicon epitaxial layer 16 provided over a p-type substrate 51 and having a well of p-type material 20. An n+ type region 26 is formed as part of a photodiode photosensor with a p-type layer 53 above it, and laterally displaced from p-well 20. In operation, photons striking the surface of the p-type layer 53 generate electrons that are collected in the n+ type region 26. A transfer gate 28 is formed between the n+ type region 26 and a second n+ type region 30 formed in p-well 20, which, when activated, transfers the photon-generated charge from the n+ type region 26 to the n+ type region 30, typically referred to as a floating diffusion region. The n+ type regions 26, 30 and transfer gate 28 form a charge transfer transistor 29 which is controlled by a transfer signal TX (not shown). The floating diffusion region 30 passes the photon-generated charge accumulated thereat to the gate of a source follower transistor 36.
A reset gate 32 is also formed adjacent and between floating diffusion region 30 and another n+ type region 34 (also formed in p-well 20). The reset gate 32, floating diffusion region 30, and n+ type region 34 form a reset transistor 31, which is controlled by a reset signal RST (not shown). The n+ type region 34 is coupled to voltage source Vaa-pix (not shown) The transfer and reset transistors 29, 31 are n-channel transistors as described in this implementation of a CMOS imager circuit.
Each pixel cell 52, 54, 56 also includes two additional n-channel transistors, a source follower transistor 36 and a row select transistor 38 (shown electrically, not in cross section). Transistors 36, 38 are coupled in series, source to drain, with the source of transistor 36 also coupled to voltage source Vaa-pix (not shown) and the drain of transistor 38 coupled to a column line 39. The drain of the row select transistor 38 is connected via a conductor to the drains of similar row select transistors for other pixels in a given pixel column. Thus, the red, green, and blue pixel cells 52, 54, 56 operate in a similar way, except that the information provided by each pixel cell 52, 54, 56 is limited by the intensities of red, green, and blue light, respectively.
One of the drawbacks of using a conventional color pixel, such as the color pixel cell array 50, is that the wavelength range corresponding to the color blue is not fully captured. This is a result of the fact that the wavelengths of light corresponding to blue light are lower than wavelengths of light for both green and red in the natural environment. Cross talk is another drawback of conventional imagers. Cross talk relates to the amount of response a pixel cell (e.g., pixel cells 52, 54, 56) exhibits for a particular wavelength of light other than the wavelength of light which it is intended to capture.
Accordingly, a pixel cell for use in an imager that exhibits improved color separation and reduced cross talk is needed. A method of fabricating a pixel cell exhibiting these improvements is also needed.